A control of this type for indicating centrally on a knob for example a selected temperature, a distribution of air or a fan speed in an air-conditioning system is already known, through for example GB 2 376 284 A. The knob in this control comprises a transparent disc, upon which the symbols are arranged, that is fixed concentrically on an inner end of the knob, and a magnification lens positioned centrally in the knob, that can display a relevant illuminated symbol on the disc at an external end of the knob. The knob thus consists of three parts, with the lens constituting a relatively large part, as far as volume is concerned, for magnifying the symbols at the external end of the knob. This design also requires the knob to have a relatively large dimension in its radial direction.